Variable Optical Attenuators (VOAs) are common components used in optical communication networks. One common VOA technology is based on an Electro-Static (ES) Micro-Electro-Mechanical (MEMS) chip. The ES-VOA component has an input optical fiber, a lens, a MEMS tilting minor and an output optical fiber. The lens focuses the input light onto the MEMS tilting mirror, and the reflected light is directed towards the output fiber. A voltage is applied to the MEMS chip, the voltage amplitude controls the mirror tilt angle. By varying the voltage and minor tilt angle, the position of the reflected spot on the output fiber is varied. With the output spot aligned to the center of output fiber core, the attenuation is minimum and limited only by the insertion loss (typically ˜0.5 dB). As the output spot of the beam of reflected light is misaligned relative to the output fiber core, the amount of light launched into the output fiber core is reduced (attenuated), and correspondingly more light is launched into the fiber cladding, and a higher level of attenuation is achieved. The maximum attenuation can be 30 dB and higher, mainly limited by the tilt range of the mirror.
There exist multiple other VOA technologies, each have advantages and disadvantages. Examples include motor-controlled vane attenuator, thermal MEMS-controlled attenuator, Mach-Zehnder attenuators, Electro-Absorption attenuators, liquid-crystal attenuators. The main advantages of the ES-VOA is rapid switching time (<2 ms), compact size (5.56 mm diameter package), low cost, low power dissipation, and high dynamic range (>30 dB). These advantages have made the ES-VOA the most common VOA solution in optical fiber networks applications.
When using VOAs in Dense Wavelength Division Multiplexing (DWDM) optical networks, there are two critical performance parameters that must be minimized: wavelength-dependent loss (WDL) and the polarization-dependent loss (PDL). The WDL refers to the variation in attenuation loss over the specified wavelength range. The PDL refers to the variation in attenuation loss over all states of input polarization. In a VOA, WDL and PDL can vary as a function of the attenuation level. WDL and PDL are undesirable because they contribute to increasing differences in optical power between wavelength channels, which in turn increases the need for channel power equalization and increases the cost and complexity of optical networks. Various design approaches have been proposed to reduce WDL in ES-VOA, see for example U.S. Pat. No. 7,295,748.
In a further development the ES-VOA component is packaged inside a Small Form factor Pluggable (SFP) housing. This product is referred to as SFP VOA. The SFP VOA offers several advantages compared to the stand-alone pigtailed ES-VOA component described above: (1) the SFP VOA is pluggable, the customer can gradually populate SFP VOA slots on the host system board as the system capacity is increased, (2) the SFP VOA pluggability allows for easy replacement, (3) no fiber management is required since the SFP VOA is connectorized, (4) the interface is digital and the attenuation level is set by a firmware instruction from the host board, the customer does not need to design control and drive hardware and does not need to know the specific characteristics of the ES-VOA component.
However, compared to the stand-alone pigtailed ES-VOA, the SFP VOA suffers from higher WDL and higher PDL. The inventors have investigated the possibility that this may be caused by modal interference between the fundamental mode and co-propagating cladding modes launched in the output fiber.
U.S. Pat. No. No. 6,498,888 issued Dec. 24, 2002 to the Institut National D'Optique, discloses a high attenuation fiber with cladding mode suppression. The attenuation mechanism in this disclosure is a cobalt doped core. A double clad fiber absorption attenuator is used to suppress cladding modes. Because the fiber is short, it can support light propagation in high order modes over its short length. When the short fiber attenuator is coupled (spliced) to a fiber, most of the light (e.g. 99%) is launched into the lowest order mode of the short fiber, but because of misalignment some light will be launched in some higher order modes. When the fiber is spliced again through misalignment these high order modes can be coupled back into the core where they would interfere with the lowest order mode. The misalignment in the splice joints is small, and therefore very little light is coupled into a cladding mode. This small amount of cladding modes is suppressed by the double clad fiber.
In a SFP VOA operating at high attenuation, the majority of the light (>99%) will be propagating in the cladding modes, whereas in the application described in U.S. Pat. No. 6,498,888 the majority of the light (>99%) is propagating in the fiber fundamental mode. It is not clear from the teaching of the prior art that a double clad fiber will work to eliminate modal noise where the attenuation is achieved by coupling large amounts of light in the fiber cladding by the ES-VOA. This large amount of light propagating in the cladding mode can couple back into the core of the single mode output fiber and interfere with light that propagated in the fiber lowest order mode resulting in modal noise.
Furthermore, the absorptive attenuation means of the 4,498,888 patent is polarization independent. This offers no teaching for mitigating the PDL degradation experienced by the SFP VOA. There are two factors that contribute to PDL degradation in the SFP VOA. The first factor relates to the variations in polarization states of the cladding modes. As the attenuation is increased, there is increasing optical power in the cladding modes, and since the propagation of cladding modes is not guided, this results in the cladding modes having a variety of polarization states. Accordingly, the amplitude of the modal interference at the core-to-core (between low-order mode in the core and high-order cladding modes) will vary as a function of the cladding modes polarization states, thereby increasing PDL. At high attenuation, given that most of the optical power is in the cladding modes, this effect leads to significant PDL degradation. A second factor is related to the polarization-dependent coupling of the light reflected from the minor coupling into the core. As the attenuation is increased, the beam offset increases, and it would be expected that the coupling of the light into the core would be different depending on whether the input polarization is parallel to the offset plane versus perpendicular to the offset plane. This difference in coupling would also contribute to PDL degradation.
Given the advantages of the SFP-VOA it is highly desirable to mitigate the PDL and WDL in a SFP-VOA device.